1. Field
The invention relates to the testing of adhesive bond strength of composite materials. The invention also relates to the use of a sonic method of non-destructive testing of bonded composite structures.
2. Description of the Background Art
This section describes background subject matter related to the disclosed embodiments of the present invention. There is no intention, either express or implied, that the background art discussed in this section legally constitutes prior art.
The use of composite bonded joints to replace fasteners on primary load structures used in aircraft has been shown to have a significant impact on reducing the cost of manufacturing airframes. Composite bonded joints are also commonly used in the automobile and boating industries. Repair of composite structures which are used in such industries is an important need. To determine whether the bond in a bonded structure has failed or been reduced in strength, so that a repair is needed, requires a method of non-destructive testing of the bonded structures. One of the barriers preventing the use of bonded composite structures is the risk that a “kissing bond” (i.e., an adhesion bond that is not broken, but is well below the required design strength). There is a need for a non-destructive method of determining whether a bond within a composite meets the required design strength. Conventional nondestructive evaluation techniques are not capable of identifying a ‘kissing bond”. Even with respect to newly fabricated bonded composite structures, destructive testing of specimens prepared simultaneously in a side-by-side fabrication with the composite article does not ensure that all of the bonds within a fabricated part meet design requirements.
One method of detecting “kissing bonds” would be to produce a negative ultrasonic pulse of such amplitude that it is capable of exceeding the minimal acceptable yield point of the composite bonds. If the bond is above the minimum required design strength and remains intact, it is considered an acceptable bond.
To cause delamination of bonds which do not meet a typical minimum requirement for an aerospace/airframe industry application, for example, it is estimated that a sound pressure greater than about 20 MegaPascal or 2900 psi would be required. In addition, the sound pressure wave would need to be a negative pressure or rarefaction wave to effectively stress the adhesive bond.
There are currently devices that deliver high power sonic energy in the form of a shock or positive pressure compression wave, which is typically applied to the front surface of the material to be tested. This compression pressure wave must then pass completely through the outer, front composite panel, through the adhesive bond and continue through the inner, rear composite panel, and then reflect from the rear surface of the inner, rear panel, reversing the polarity of the wave, to return back to the adhesive interface as a rarefaction or negative pressure wave. A usable bounce would have to occur from a surface that is perpendicular to the incident wave and be at a lower acoustic impedance to produce the desired rarefaction negative pressure wave at the location of the adhesive bond.
One obvious disadvantage of delivering a compression wave is that the sound must travel much further to bounce off the back surface of the inner, rear panel, resulting in less wave intensity due to the increased propagation distance. A major disadvantage of using the compressive wave is that usable areas for testing load bearing structures is limited to structures where the rear surface is parallel to the front surface, and where the rear interface is air or other low acoustic impedance material. If the front, outer composite panel (nearest to entry of the sound wave) is bonded to an inner structure which is metal (higher acoustic impedance), then the reflected wave is a compression, positive pressure wave and not a usable stress wave for non-destructive testing.
Although there are ways for existing sonic devices to create a rarefaction wave, the coupling of a negative pressure sound wave into the material to be tested becomes a significant problem, since the sonic device must not pull away or lose contact with the surface of the material under test.